With the development of modern communication technology and the improvement of people's living standard, smart phones are provided with various functions, such as making calls, sending messages, shopping, booking tickets, etc. Meanwhile, too much important information is stored in smart phones, such as contact phone numbers, content of messages, various accounts and passwords, and other information.
Because smart phones are expensive, they become targets in the eyes of thieves. Recently, loss of smart phones has become increasingly common. Many users have experienced loss of smart phones, e.g., being stolen by thieves. When the users' smart phones are stolen, it can cause trouble and/or enormous economic loss to the users.